Semi-Identical Twins Born from One Egg Fertilized by Two Sperm

February 28, 2019 12:49 PM

Because an ultrasound taken early in the pregnancy showed both fetuses shared the same placenta, doctors assumed the fetuses were identical twins. But when an ultrasound eight weeks later revealed that one child was male and the other female, something considered impossible for identical twins, the Gabbett team knew something extraordinary had happened.

New England Journal of Medicine

posted by If only I had a penguin... (17 comments total)

I had heard about this years ago, but over the years had assumed it was conjecture. I know twins who at first glance appear identical, but who are relatively easy to tell apart. So would it be possible for these semi-identical twins to share identical genes from the mother, but non-identical genes from the father, or even possibly genes from two different fathers?
posted by weapons-grade pandemonium at 1:16 PM on February 28, 2019

I read this article yesterday, explain to me why they had to yank the ovaries out of a three year old girl? That just struck me so badly, like when they used to think they could randomly change children's gender to suit the parents. So if someone here can explain this, I would appreciate it.
posted by Oyéah at 1:18 PM on February 28, 2019 [2 favorites]

The original assumption of identical-ness came from the sharing of a single placenta, which is the result of a single egg. Two eggs, leading to fraternal twins, create two placentae.

(It's also possible for identical twins to have two placentae, but that's the result of the egg splitting very, very early in the process, within a day or two of fertilization)
posted by hanov3r at 1:20 PM on February 28, 2019

To me it reads that doctors thought the ovaries were cancerous or pre-cancerous.
posted by wellred at 1:21 PM on February 28, 2019 [6 favorites]

I read this article yesterday, explain to me why they had to yank the ovaries out of a three year old girl? That just struck me so badly, like when they used to think they could randomly change children's gender to suit the parents. So if someone here can explain this, I would appreciate it.

I lack the background knowledge to fully understand the NEJM article but from the little I saw, something about the way the genes combined made them think these kids would be at high risk for reproductive cancers. They were monitoring the kids and the girl shows "changes in her ovaries" that concerned them so they removed her ovaries, presumably so she wouldn't get ovarian cancer. They continue to monitor the boy's testes.
posted by If only I had a penguin... at 1:22 PM on February 28, 2019 [4 favorites]

explain to me why they had to yank the ovaries out of a three year old girl?

The doctors were concerned that the children might experience reproductive cancers at a higher than expected rate compared with other twins, so they were monitoring them both carefully, and the girl had "changes in her ovary that people weren't comfortable with," which is doctor code for pre-cancerous or the potential for things to go badly very, very fast.

I didn't at all get the impression that they removed her ovaries on a whim.
posted by cooker girl at 1:22 PM on February 28, 2019 [11 favorites]

Oyéah, per Gizmodo: She was also later diagnosed with gonadal dysgenesis, a congenital condition linked to cases of rare chromosome configurations. The disorder causes a person’s genitalia to not develop normally and to have a high chance of becoming cancerous, so the twin received a precautionary surgery to remove her ovaries.
posted by Iris Gambol at 1:24 PM on February 28, 2019

I know twins who at first glance appear identical, but who are relatively easy to tell apart. So would it be possible for these semi-identical twins to share identical genes from the mother, but non-identical genes from the father, or even possibly genes from two different fathers?

Probably not:

To see if the phenomenon might be more common than doctors believed, the Gabbett team examined an international database of 968 fraternal twins and their parents. None showed the same pattern.

It is possible that the twins you know are actually same-sex fraternal twins who just appear very similar (I know a set of fraternal triplet girls who looked "exactly alike" to strangers). But probably they're just "identical twins" ie monozygotic that just don't look exactly alike. DNA is just a blueprint.
posted by muddgirl at 1:39 PM on February 28, 2019

The researchers say it appears that after fertilization, the DNA from the egg and two sperm divided, then got divvied up to create three embryos [one of which was nonviable].

This makes it sound as if, potentially, a single person could be the product of one egg and two sperm. Anyone who knows more about this stuff have thoughts on that?
posted by showbiz_liz at 1:40 PM on February 28, 2019

I'd think possible (Genetic Chimera) but extremely unlikely.
posted by zinon at 1:58 PM on February 28, 2019 [1 favorite]

Two sperm (but still the same father) is dispermy, and can result in triploidy (fetus with an extra set of chromosomes).
One egg, sperm from more than father = the plot hinge from the gruesome X-Files episode "Home."

(Edited, misuse of polyspermy term)
posted by Iris Gambol at 2:00 PM on February 28, 2019 [1 favorite]

In particular I am 46 / 47 XY / XXY klinefelter mosaic intersex, which is an uncommon form of Klinefelter's syndrome.

Extremely uncommon! None the less, this FPP shows that there are many different extremely rare conditions - and not just ones which lead to intersex. We should be humbled in the face of such diversity; there are so many different ways to be human.
posted by Joe in Australia at 5:05 PM on February 28, 2019 [7 favorites]

I read this article yesterday, explain to me why they had to yank the ovaries out of a three year old girl? That just struck me so badly, like when they used to think they could randomly change children's gender to suit the parents. So if someone here can explain this, I would appreciate it.

It was my understanding that they were discussing the twins from the 2007 case, though I may be mistaken. If so, the girl may have had abrupt changes as she starts to hit puberty, which could be why they removed her ovaries.
posted by annieb at 6:11 PM on February 28, 2019

It is possible that the twins you know are actually same-sex fraternal twins who just appear very similar (I know a set of fraternal triplet girls who looked "exactly alike" to strangers).

Mary-Kate and Ashley Olsen are fraternal twins.
posted by Autumnheart at 7:09 PM on February 28, 2019 [2 favorites]

How was there enough generic material in the egg to create 2 fraternal-ish embryos? Is that implying that if only 1 sperm had reached the egg, the embryo would have an entire extra set of chromosomes?
posted by picklenickle at 9:00 AM on March 3, 2019

The video at the end of this Boingboing article suggests that the the nuclei formed a "tripolar spindle apparatus" resulting in two viable cells and one unviable cell. However this was not the proximate cause of twinning; the twinning occurred later and the twins are chimeric, both having a complement of cell lines from each of the two initial viable cells.
posted by sjswitzer at 9:05 AM on March 5, 2019

Human genetics really does amaze me sometime. A single wrong base pair can lead to a non-viable embryo in some circumstances, but then you can get craziness like this. Three halves of gene sets mash up, turn into two viable ones, which fuse to make a viable blastocyst which then splits, resulting in a pair of chimeras.
posted by tavella at 11:27 AM on March 5, 2019